1. Field of the Invention
The present invention relates to an information memory medium making use of a polymeric liquid crystal. More particularly, it relates to an information memory medium utilizing a ferroelectric chiral smectic phase of a polymeric liquid crystal. It also relates to an information recording/holding process making use of such a medium.
2. Related Background Art
Optical recording methods have been hitherto put into practical use as methods that can provide a large capacity and can achieve superior random access. In particular, researches on rewritable type optical disks are being pushed forward, and it is attempted to put into practical use those utilizing photomagnetic effect and those utilizing phase changes. Of these mediums, polymeric liquid crystals are proposed also as materials for information memory mediums (Japanese Patent Applications Laid-open No. 59-10930, No. 59-35989 and No. 62-154340). In particular, as a recording method, a method is proposed in which spiral pitch lengths of a cholesteric polymeric liquid crystal are changed or pits are formed in the state of no alignment to change optical reflectance in multi value (Japanese Patent Applications Laid-open No. 62-107448 and No. 62-12937).
The above information memory mediums making use of polymeric liquid crystals, however, have so low an erasing speed that it has been questioned for them to be put into practical use. In order to solve such a problem, for example, information memory mediums utilizing a ferroelectric polymeric liquid crystal are proposed (Japanese Patent Applications Laid-open No. 63-153520, No. 63-266647 and No. 63-271228). These utilize glass transition points for carrying out the information recording/holding process.
On the other hand, taking note of high-speed response, an information memory medium utilizing surface stabilization effect of a low-molecular ferroelectric liquid crystal is also proposed (Journal of Applied Physics, 67, p.996, 1990).
The above conventional techniques, however, have been disadvantageous in that, as in the former techniques, the electric-field response speed is lowered and no high response speed can be sufficiently achieved when the glass transition temperature of the ferroelectric liquid crystal is made sufficiently higher than the room temperature so that recorded data can be stably held.
In the latter technique in which the surface stabilization effect of a low-molecular ferroelectric liquid crystal is utilized, no recording layer can be retained unless the medium has a cell structure, and also the cell must be constructed with a cell thickness of about 1 .mu.m. This has brought about a difficult problem on the manufacture. In addition, the recorded data are unstable compared with those of the mediums in which the polymeric liquid crystals are used, and hence unsuitable for their long-term storage, disadvantageously.